2 * SPDX-License-Identifier: BSD-2-Clause
4 * Copyright (c) 2005 John Baldwin <jhb@FreeBSD.org>
6 * Redistribution and use in source and binary forms, with or without
7 * modification, are permitted provided that the following conditions
9 * 1. Redistributions of source code must retain the above copyright
10 * notice, this list of conditions and the following disclaimer.
11 * 2. Redistributions in binary form must reproduce the above copyright
12 * notice, this list of conditions and the following disclaimer in the
13 * documentation and/or other materials provided with the distribution.
15 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
16 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
17 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
18 * ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
19 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
20 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
21 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
22 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
23 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
24 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
28 #ifndef __SYS_REFCOUNT_H__
29 #define __SYS_REFCOUNT_H__
31 #include <machine/atomic.h>
33 #if defined(_KERNEL) || defined(_STANDALONE)
34 #include <sys/systm.h>
37 #define KASSERT(exp, msg) /* */
40 #define REFCOUNT_SATURATED(val) (((val) & (1U << 31)) != 0)
41 #define REFCOUNT_SATURATION_VALUE (3U << 30)
44 * Attempt to handle reference count overflow and underflow. Force the counter
45 * to stay at the saturation value so that a counter overflow cannot trigger
46 * destruction of the containing object and instead leads to a less harmful
50 _refcount_update_saturated(volatile u_int *count)
53 panic("refcount %p wraparound", count);
55 atomic_store_int(count, REFCOUNT_SATURATION_VALUE);
60 refcount_init(volatile u_int *count, u_int value)
62 KASSERT(!REFCOUNT_SATURATED(value),
63 ("invalid initial refcount value %u", value));
64 atomic_store_int(count, value);
68 refcount_load(volatile u_int *count)
70 return (atomic_load_int(count));
74 refcount_acquire(volatile u_int *count)
78 old = atomic_fetchadd_int(count, 1);
79 if (__predict_false(REFCOUNT_SATURATED(old)))
80 _refcount_update_saturated(count);
86 refcount_acquiren(volatile u_int *count, u_int n)
90 KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
91 ("refcount_acquiren: n=%u too large", n));
92 old = atomic_fetchadd_int(count, n);
93 if (__predict_false(REFCOUNT_SATURATED(old)))
94 _refcount_update_saturated(count);
99 static __inline __result_use_check bool
100 refcount_acquire_checked(volatile u_int *count)
104 old = atomic_load_int(count);
106 if (__predict_false(REFCOUNT_SATURATED(old + 1)))
108 if (__predict_true(atomic_fcmpset_int(count, &old,
115 * This functions returns non-zero if the refcount was
116 * incremented. Else zero is returned.
118 static __inline __result_use_check bool
119 refcount_acquire_if_gt(volatile u_int *count, u_int n)
123 old = atomic_load_int(count);
127 if (__predict_false(REFCOUNT_SATURATED(old)))
129 if (atomic_fcmpset_int(count, &old, old + 1))
134 static __inline __result_use_check bool
135 refcount_acquire_if_not_zero(volatile u_int *count)
138 return (refcount_acquire_if_gt(count, 0));
142 refcount_releasen(volatile u_int *count, u_int n)
146 KASSERT(n < REFCOUNT_SATURATION_VALUE / 2,
147 ("refcount_releasen: n=%u too large", n));
149 atomic_thread_fence_rel();
150 old = atomic_fetchadd_int(count, -n);
151 if (__predict_false(old < n || REFCOUNT_SATURATED(old))) {
152 _refcount_update_saturated(count);
159 * Last reference. Signal the user to call the destructor.
161 * Ensure that the destructor sees all updates. This synchronizes with
162 * release fences from all routines which drop the count.
164 atomic_thread_fence_acq();
169 refcount_release(volatile u_int *count)
172 return (refcount_releasen(count, 1));
175 #define _refcount_release_if_cond(cond, name) \
176 static __inline __result_use_check bool \
177 _refcount_release_if_##name(volatile u_int *count, u_int n) \
181 KASSERT(n > 0, ("%s: zero increment", __func__)); \
182 old = atomic_load_int(count); \
186 if (__predict_false(REFCOUNT_SATURATED(old))) \
188 if (atomic_fcmpset_rel_int(count, &old, old - 1)) \
192 _refcount_release_if_cond(old > n, gt)
193 _refcount_release_if_cond(old == n, eq)
195 static __inline __result_use_check bool
196 refcount_release_if_gt(volatile u_int *count, u_int n)
199 return (_refcount_release_if_gt(count, n));
202 static __inline __result_use_check bool
203 refcount_release_if_last(volatile u_int *count)
206 if (_refcount_release_if_eq(count, 1)) {
207 /* See the comment in refcount_releasen(). */
208 atomic_thread_fence_acq();
214 static __inline __result_use_check bool
215 refcount_release_if_not_last(volatile u_int *count)
218 return (_refcount_release_if_gt(count, 1));
221 #endif /* !__SYS_REFCOUNT_H__ */